The migration and development of mobile phones continues towards multiband applications. It is expected that these multiband applications could require a combination of several various or complicated modulation methods in the transceiver, such as, for example, Enhanced Data Rates for Global Evolution (EDGE) frequency modulation and amplitude modulation. Further complicating matters is the trend in mobile phones toward increased integration of features and services. This drive towards higher integration has led to a situation where most of the functions are now integrated in one or more RFICs to reduce the size or avoid increases in the size of the mobile phone. These functions include for example, up/down converters, LNAs, Phase Locked Loops (PLL), amplifiers/buffers, and other such functionalities in devices and apparatus used in wireless communication systems. Depending on the selected radio architecture, multiple different oscillators are needed in a transceiver to provide down/up conversion, channel selection and modulation for a given system and modulation method.
Currently known multiband RF transceivers such as the type utilized in cellular communication systems typically use crystal resonator oscillators. The number of different crystal resonator oscillators is the main source of transceiver cost and size, and make the greatest contribution to performance. Generally, the crystal resonators are packaged separately and mounted on the motherboard as separate components or modules for the oscillator. The crystal resonator packages typically also include the tuning and amplifying electronics for the oscillator. The crystal resonator packages add weight to the transceiver and the crystal and resonating components are susceptible to damage from shock or impact forces if the transceiver is dropped due to the mass of the package and the component mounting and attachment means' ability to withstand such forces. One approach to reducing the size of the transceiver is to select an RF architecture such as direct conversion and modulation methods which use a minimum number of oscillators and thus can save space and volume compared to other RF architectures. Attempts at further integration typically integrate the amplifier functions of the reference oscillators on the same chips with the PLLs, however, the crystal itself is left as an external component, i.e., not integrated with the PLL chips. Further, some of the required component functions, such as from, crystals and resonators do not lend themselves to and are often very difficult to miniaturize, hybridize and integrate due primarily to incompatibility with the most often used semiconductor such as, silicon and gallium arsenide technologies in the integration.
Crystal based oscillators are preferred in multiband radio frequency transceiver because they provide a high Q value for the reference oscillator function. As supporting component functions such as phase detectors in PLL's operate at much higher frequencies due to development, improvement and implementation of integrated circuit technologies, the reference oscillator frequencies are likewise required to increase in frequency. It is expected that to achieve the higher frequencies and size reduction objectives, it will be increasingly difficult to manufacture crystals that will need to be physically very small, light and have tight tolerance characteristics. Further difficulties accompany the mechanical manufacturing aspects, such as, crystal lapping, precision cutting and contact joining for example, by wire or lead bonding/joining techniques.
Another shortcoming is the packaging methods such as wire bonding typically used in the packaging of the reference crystals or crystals in the reference oscillator modules leads to calibration requirements during and/or after production of the transceiver or the reference oscillator module. Typically, this calibration is made with help of switchable capacitors for use in adjusting the center frequency of the oscillator. Also, inaccuracy of low Q value resonators used in VCO's may require center frequency calibration in the production of the VCO oscillator modules. The reader is referred to textbooks and technical journals available in the literature for further detailed information relating to the design issues relating to performance and for additional explanations of the limitations of crystals and crystal oscillators and their respective operational limitations.
Additionally, the monolithic integration of high Q (>10000) value resonators has not been technically feasible due to different material compatibility requirements such as, for example, between silicon which to date is the most widely used media for IC integration and quartz as found in crystals, or SAW resonators on lithium niobate or lithium tantalate to name a few.
It would be desirable therefore to provide a crystal-less resonator structure and a method of implementing resonators for use in place of crystals in VCOs and reference oscillators including voltage controlled, temperature controlled crystal oscillators (VC(TC)XOs). It is desirable that the crystal-less resonator structure be implemented as thin film based resonators (FBAR, BAW) integrated or flip chipped on top of a RFIC or RFICs which may possibly include a phase-locked-loop (PLL) as used in TDMA systems or possibly multiple PLL chips which would be needed for example, in transceivers having simultaneous transmit and receive capability as required in CDMA systems. Integrating additional RF band functionality, such as for example, global positioning system (GPS) capability into the transceiver may require one or more additional oscillator/crystal functions which further increase cost and complexity of the transceiver. It is desirable therefore to also use crystal-less resonator structures to implement such additional RF band functionalities.
It is an object therefore of the present invention to provide a highly miniaturized RF transceiver based on RFIC or multiple RFICs possibly including a PLL or multiple PLLs including all necessary resonators in the form of FBAR (film bulk acoustic resonator) or BAW (bulk acoustic wave resonator) for the VCO or VCOs and a reference oscillator either flip chipped or monolithically integrated on the same chip with the PLL or PLLs used in the RF transceiver system.